Hydrogen is a common industrial chemical which is used in a wide variety of chemical processes. Hydrogen is also used extensively as a fuel, both for direct combustion and in fuel cells. Many of these applications, including some fuel cells, require that the hydrogen supplied to them be relatively pure.
The most common source for industrial hydrogen is a steam reformer. In a steam reformer, a gaseous fuel is mixed with steam and passed over a catalyst to produce a stream containing hydrogen, carbon dioxide, carbon monoxide, and water. In applications in which any of the products other than the hydrogen is considered an impurity, the reformer product stream must be purified. For example, carbon dioxide is harmful to alkaline fuel cells and carbon monoxide is harmful to solid polymer electrolyte fuel cells. If hydrogen from a reformer is to be fed to either of these types of fuel cells, either the carbon dioxide or the carbon monoxide must first be removed. The carbon monoxide content can be reduced by passing the product stream over another catalyst at a temperature favoring the conversion of carbon monoxide and water to produce hydrogen and carbon dioxide by the water-gas shift reaction.
Currently, there are several methods available for separating hydrogen from the impurities in order to purify a reformer product stream. However, none of these methods are entirely satisfactory. One common method for performing this separation is to use a permeable membrane. The membrane may be fabricated either from a polymer such as polyetherimide or from a metallic combination such as palladium-silver. The polymeric membranes rely on differences in the relative diffusivities of the gaseous molecules through the membrane to make the desired separation. Since hydrogen will diffuse more readily through the polymeric membranes than other molecules, polymeric membranes may be used to purify hydrogen. However, other molecules can also diffuse through the membrane, resulting in an incomplete separation of hydrogen from the other molecules. The result is a low hydrogen selectivity relative to other molecules. The relatively low hydrogen selectivities which characterize polymeric membranes make them impractical where very high purity hydrogen is required.
Metallic palladium-silver membranes provide excellent hydrogen selectivity because they transport hydrogen in its atomic state, rather than its molecular state. The diffusivities of all other molecules, which must be transported in a molecular state, are extremely low through palladium-silver membranes. However, the materials used to fabricate palladium-silver membranes are not as readily available as polymeric materials, making them undersirable to use for most hydrogen purification applications. Recently, palladium and silver have been vacuum sputtered onto polyetherimide in thin films in an effort to combine the best properties of polymeric and palladium-silver membranes. These membranes have shown promising results.
Other possible means for purifying hydrogen streams include cryogenic separation and pressure swing absorption. Both of these processes require elaborate processing equipment, which makes them unsuitable for many hydrogen purification applications.
Accordingly, there has been a continuous effort in this field of art to develop a highly selective hydrogen purification membrane which is practical for a variety of applications.